An electrolytic process is a process in which electrical energy, which is a clean energy, is utilized to conduct chemical reactions on the electrode surfaces while controlling the reactions. When applied to aqueous solution systems, the process can generate hydrogen, oxygen, ozone, hydrogen peroxide, etc. In industrial electrolysis, the process is a basic technique generally used for the electrolysis of sodium chloride, electroplating, metal collection, etc. Recently, the process is coming to be utilized for wastewater treatment because it can indirectly decompose organic pollutants or directly electrolyze the pollutants after adsorption thereof onto an electrode.
On the other hand, it is known that in electrolysis, oxidation reactions at the anode yield an oxidizing agent (e.g., effective chlorine or ozone) effective in water treatment and partly generate active species such as OH radicals. Water containing these components is in general use under the name of active water, functional water, ion water, sterilization water, etc. Recently, this water has come to be called exclusively as electrolytic water.
Although electrolytic processes are being practically used as shown above, it has been pointed out that intended reactions do not sufficiently proceed depending on the electrode materials. In general, anodic oxidation in the electrolysis of an aqueous solution yields an electrolysis product from water. However, when the electrode catalyst used has high reactivity in the electric discharge of water, there are often cases where the oxidation of coexistent substances does not readily proceed.
Examples of the electrode materials for use in electrolytic electrodes (anodes) for oxidation include lead oxide (PbO2), tin oxide (SnO2), platinum (Pt), DSAs, and carbon. In order for a material to be usable as an electrode substrate, it should have corrosion resistance so as to secure a long life and not to foul the surface to be treated. Usually, the anode feeders are limited to valve metals such as titanium and alloys thereof, and the electrode catalysts also are limited to noble metals such as platinum (Pt) and iridium (Ir) and oxides thereof. It is, however, known that even when such expensive materials are used, they are consumed depending on the density of the current applied and on the lapse of time and dissolve away in the solution. There is hence a desire for an electrode having better corrosion resistance.
Graphite and amorphous carbon materials have been used as electrode materials. However, these materials are consumable and are highly consumed especially in anodic polarization. Diamond is excellent in thermal conductivity, optical transparency, and resistance to high temperatures and oxidation, and the electrical conductivity thereof can be regulated especially by doping. Diamond is hence regarded as a promising material for use in semiconductor devices and energy conversion elements.
Swain et al. [Journal of Electrochemical Society, Vol. 141, 3382-(1994)] recently reported that diamond used as an electrode for electrochemical use is stable in an acid electrolytic solution and suggested that diamond is far superior to other carbon materials. Basic electrochemical properties are described in detail in Denkikagaku Kai-shi, p. 521, Vol. 72, No. 7 (2004).
U.S. Pat. No. 5,399,247 suggests that use of diamond as an anode material enables the decomposition of organic wastewaters. JP-A-2000-226682 proposes a method in which conductive diamond is used as an anode and a cathode to electrochemically treat an organic matter. JP-A-2000-254650 proposes a method in which a conductive diamond electrode and a gas diffusion cathode for hydrogen peroxide generation are used as an anode and a cathode, respectively, to treat water. Recently, a technique in which a diamond electrode is used for the synthesis of electrolytic water and ozone gas was disclosed (JP-A-11-269685).
It can be expected from those investigations that the electrolytic processes employing diamond as an electrode attain a higher efficiency than in processes employing conventional electrodes. However, improvements have been desired as shown below from the standpoint of practical use.
Methods which have been developed for diamond layer formation are hot-filament CVD, microwave plasma CVD, the plasma arc jet method, PVD, and the like. In the CVD synthesis method, which is a general process for producing a diamond layer, it is required that the substrate should have a thermal expansion coefficient close to that of diamond became the target layer is formed through a high-temperature reduction step conducted at about 700° C. or higher.
Semiconducting silicon which is single-crystalline or polycrystalline is generally used as the substrates of diamond electrodes because the thermal expansion coefficient thereof is close to that of diamond. However, this material has poor mechanical strength and is limited in size. It is therefore difficult to use this substrate material for size enlargement. Since the electrodes for use in industrial electrolysis have complicated shapes, it is after all preferred to employ a metallic substrate which is easy to process and has high mechanical strength. Known as electrode substrate substitutes are carbon, valve metals, Ta, Nb, Ni, Mo, W, and carbides thereof.
In the case where diamond is used in an electrode, the material of the substrate thereof preferably is one which, when the electrode is used as an anode, is stable at noble potentials and on which a stable passive-state film is formed. In particular, a valve metal can be used as a metal which is stable in acid solutions in an anode potential region. Use of a niobium or tantalum substrate is being investigated partly because of its property of being less apt to yield hydrides in a hydrogen atmosphere.
However, niobium has an unsolved problem that this metal as a substrate is expensive. Compared to niobium, tantalum is closer to diamond in the thermal expansion coefficient (6 times). Tantalum hence shows better suitability for bonding to a diamond layer and is expected to bring about improved stability. However, tantalum is even more expensive than niobium and is hence unsuitable for practical use. Furthermore, the specific gravities of niobium and tantalum are 8.56 and 16.60, respectively, and are far higher than those of other metals, e.g., 4.54 for titanium, and this has been an obstacle to the practical use of a large electrode.
On the other hand, titanium is relatively far inexpensive (from 1/10 to 1/20 the cost of niobium) and has a high value of specific strength, i.e., strength/specific gravity. Titanium can be relatively easily extended by rolling, etc., and processing techniques such as, e.g., cutting have been established. Investigations on the use of titanium as a diamond substrate are being made and this substrate is utilized. However, since diamond is generally synthesized in a high-temperature reducing hydrogen atmosphere, use of a titanium substrate has a drawback that when the titanium surface is directly exposed to the atmosphere, there are cases where a hydride generates depending on the synthesis conditions and the hydride penetrates into and is occluded in the substrate to impair the stability of the substrate. In addition, the thermal expansion coefficient of titanium (9 times) considerably differs from that of diamond.
To form an interlayer of, e.g., a carbide on the substrate surface for the purposes of adhesion and substrate protection is preferred (JP-A-9-268395). However, most of such interlayers have poor resistance to electrolytic corrosion.
For providing a stable anode, it is required to improve the durability of a substrate and maintain it.
In the DSEs for use as anodes for industrial electrolysis which employ a noble metal as a catalyst, titanium or a titanium alloy is generally used as the substrates. The effect of an interlayer has been disclosed from long ago as a basic technique for prolonging the life of noble-metal oxide electrodes (DSEs) in acid electrolytic baths (Japanese Patent No. 1296411). In the case of a diamond electrode, however, the oxide interlayer is mostly reduced because radicals of, e.g., hydrogen generate under the CVD diamond synthesis conditions, making it difficult to utilize that technique.
On the other hand, Japanese Patent No. 2761751 discloses a technique in which a thin layer of a metal or alloy composition is formed as an interlayer on a substrate having poor corrosion resistance in producing an anode for oxygen generation to thereby improve corrosion resistance. The interlayer technology has been developed with regard to DSEs, and effectiveness thereof in processes for synthesizing diamond catalyst electrodes has not yet been reported or disclosed so far.
Under these circumstances, it has been required to develop a diamond electrode which has durability, can be easily made to have a large size, and can be utilized for industrial electrolysis.